Methods and systems for heating thermal storage units

ABSTRACT

Methods and systems for heating a thermal storage unit (TSU) are provided. A thermal storage system is provided that includes a system of heaters removably disposed at least partially within the TSU, a control system for adjusting power provided to the heaters, and a removal tool for removing one or more of the heaters from the TSU when the TSU is still hot. The thermal storage system may be used in a thermal and compressed air storage system for backup power applications.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/738,825 (hereinafter “the '825 patent application”),(Attorney Docket No. AP-46), filed Dec. 16, 2003, entitled “ThermalStorage Unit and Methods for Using the Same to Heat a Fluid,” theentirety of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to methods and systems for heating thermalstorage units (TSUs) and managing a system of heaters in a manner thatincreases the operational life of the heaters and maintains the TSUs atdesired operating conditions with less interruption.

BACKGROUND OF THE INVENTION

TSUs are well-known and often used in power delivery systems, such ascompressed air storage (CAS) systems and thermal and compressed airstorage (TACAS) systems. Such systems, often used to provide anavailable source of electrical power, often use compressed air to drivea turbine that powers an electrical generator.

In TACAS systems, it is desirable to heat the compressed air prior toreaching the inlet port of the turbine. It is known that heated air, asopposed to ambient or cool air, enables the turbine to operate moreefficiently. Therefore, a mechanism or system is needed to heat the airbefore providing it to the turbine. One approach is to use a TSU.

TSUs provide thermal mass for energy storage. Once a TSU is heated to adesired temperature, fluid, such as compressed air, may be heated byrouting the fluid through the TSU. Convection transfers heat from theTSU's thermal mass to the fluid, raising the temperature of the fluid asit passes through the TSU. Illustrative TSUs are described, e.g., in the'825 patent application.

A TSU in a TACAS system for backup power applications, such as thatdescribed in the '825 patent application and in U.S. patent applicationSer. No. 10/361,728, (Attorney Docket No. AP-44), filed Feb. 5, 2003,entitled “Systems and Methods for Providing Backup Energy to a Load,”the entirety of which is incorporated herein by reference, preferably ismaintained at its operating temperature continuously during a standbymode of operation for the system to deliver the rated power. Thecriteria for selecting a heating system, including the heatercontroller, is based on reliability and cost over the product life,which typically is 20 years. The following considerations are taken intoaccount:

-   -   (1) operational life of the heater;    -   (2) the nature of heater failure and the impact on the operation        of the heating system;    -   (3) difficulty of heater replacement and the impact of        replacement on the ability of the backup power system to protect        the load;    -   (4) integration and packaging of the heaters with the insulation        system and the impact on the overall size of the TSU assembly;    -   (5) impact on standby losses the TSU assembly experiences when        it is not delivering backup energy; and    -   (6) product availability—that is, whether the heating system is        standard or custom.

In the past, TSUs largely have been heated with radiant heaters, whichare disposed external to the TSUs. Disadvantageously, radiant heatersmay waste a lot of energy by emanating heat to the ambient environmentoutside the TSUs, resulting in high standby losses. To reduce this typeof standby loss, the radiant heater may be encased in thick insulation.This, however, occupies valuable space in a TACAS system, in which spaceis at a premium. Radiant heaters also are difficult to repair, requiringremoval of the thick insulation surrounding the radiant heater andrequiring additional personnel and/or special tools to maneuver thenearly half ton TSU out of the TACAS cabinet. When a radiant heatermalfunctions, there often is significant loss in the overall uniformityof the temperature in the TSU, thereby requiring the TACAS system to beshutdown immediately for repair. This typically requires several daysfor a well-insulated TSU system to cool down to a safe handlingtemperature. During this time period, the TACAS system is offline andunable to provide backup power.

In view of the foregoing, it would be desirable to be able to providemethods and systems for heating a TSU with compact size and reducedstandby losses.

It further would be desirable to be able to provide methods and systemsfor easily replacing and repairing the heating system of a TSU assemblywithout requiring additional personnel and/or special tools.

It even further would be desirable to be able to provide methods andsystems for continuously heating a TSU without significant loss in theoverall uniformity of the temperature of the TSU even when the heatingsystem malfunctions.

It also would be desirable to be able to provide methods and systems forreducing the amount of time that the TACAS system is offline whenrepairing and/or replacing the heating system.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide methods and systems for heating a TSU with compact size andreduced standby losses.

It further is an object of the present invention to provide methods andsystems for easily replacing and repairing the heating system of a TSUassembly without requiring additional personnel and/or special tools.

It even further is an object of the present invention to provide methodsand systems for continuously heating a TSU without significant loss inthe overall uniformity of the temperature of the TSU even when theheating system malfunctions.

It also is an object of the present invention to provide methods andsystems for reducing the amount of time that the TACAS system is offlinewhen repairing and/or replacing the heating system.

These and other objects of the present invention are accomplished by aTSU heating system preferably comprising a plurality of resistivecartridge heaters removably disposed in bores that are uniformlydistributed throughout the thermal storage mass of the TSU. This reducesstandby losses since the heat from the resistive cartridge heaters flowsdirectly into and through the thermal storage mass of the TSU beforepassing through the insulation of the TSU and into the ambientenvironment.

In one embodiment, the heating system of the present invention comprisesone or more redundant heaters, which are heaters in excess of a minimumnumber of heaters needed to heat/reheat the TSU within specification.More specifically, the minimum number of heaters is that quantity neededto raise a characteristic temperature of the TSU to a predeterminedvalue in a predetermined amount of time when the minimum number ofheaters are operated at the maximum power permitted by a heater controlprogram. The redundant heaters shortens the time needed to reheat theTSU after a discharge event in which the temperature of the TSU isreduced and enables the heating system to operate within specificationeven with the failure of one or more heaters. That is, the redundantheaters allow the system to operate continuously without significantloss in the overall uniformity of the temperature of the TSU, even whenone or more of the heaters have failed. Furthermore, when more than theminimum number of heaters are operational, the load on each individualheater is reduced, thereby extending its operational life. The use ofredundant heaters also permits replacement of failed heaters to bedeferred to routine TACAS maintenance intervals, rather than requiringimmediate repair or replacement when an individual heater malfunctions.

The heaters may be configured so that a heater that experiences failureautomatically disconnects from the heater network without affecting theother individual heaters, thereby permitting uninterrupted operation ofthe TSU system. A sensor coupled to the heaters detects when failure ofany individual heater or heaters occur, thereby permitting a controllerto adjust control parameters for uninterrupted operation and to generatewarnings and alarms for maintenance.

In one embodiment, the present invention also utilizes a control systemthat requires no more than a single temperature input signal to reliablycontrol the operation of the heater system. Preferably, the singletemperature input signal to the heater control system comprises anaverage temperature signal derived from a plurality of temperaturesensors positioned at numerous thermally equivalent locations over theTSU.

In one embodiment, the TSU assembly is disposed in the TACAS cabinet foreasy access to the cartridge heaters. Systems and methods are providedto remove one or more cartridge heaters from the TSU assembly while theTSU assembly is still hot. This reduces the amount of time the TACASunit is offline and the backup power system is unable to provide backuppower.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the present invention, its nature and variousadvantages will be more apparent from the accompanying drawings and thefollowing detailed description, in which:

FIG. 1 illustrates one embodiment of a TSU assembly of the presentinvention comprising a plurality of heaters removably disposed within aTSU;

FIG. 2 illustrates a typical construction of an electrical resistancecartridge heater;

FIG. 3 illustrates the TSU system of the present invention comprisingthe TSU assembly of FIG. 1 coupled to a controller;

FIG. 4A is a simplified block diagram of a backup power system, which isone application that can use the TSU system of the present invention;

FIG. 4B illustrates a system cabinet having the TSU system of thepresent invention;

FIG. 5A illustrates an example of characteristic heating curves of a TSUassembly of the present invention;

FIG. 5B illustrates the power provided to a heating system of thepresent invention to heat a TSU assembly in accordance with thecharacteristic heating curves of FIG. 5A;

FIG. 6 illustrates a removal tool and storage/transport container forremoving and storing/transporting cartridge heaters; and

FIGS. 7A-G illustrates various embodiments of couplers incorporated inthe removal tool of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an illustrative TSU assembly of the presentinvention is described. TSU assembly 10 comprises TSU 12 within whichare disposed flow channels 14 a-d and a plurality of heaters 16, e.g., aplurality of cartridge heaters. Insulation (not shown) can be disposedaround TSU 12 to increase the efficiency of TSU assembly 10. In oneembodiment, flow channels 14 a-d are coupled together in fluidiccommunication. Fluid, such as compressed air, enters TSU 12 via inletport 18, flows through flow channels 14 a-d, and exits TSU 12 via outletport 20. Fluid may flow sequentially through each flow channel, inparallel through two or more flow channels, or any other combination offlow channels. Alternatively, fluid may enter TSU 12 via port 20 andexit via port 18. Alternatively, TSU 12 may comprise one or moreindependent flow channels that are not coupled in fluidiccommunication—e.g., fluid entering each flow channel 14 a-d exits thatflow channel without flowing through any of the other flow channels. Amore detailed description of TSU 12 may be found in commonly-assignedU.S. patent application No. ______ (Attorney Docket No. AP-46 CIP),entitled “Thermal Storage Unit and Methods for Using the Same to Heat aFluid,” filed on Sep. 17, 2004, the entirety of which is incorporatedherein by reference.

TSU 12 may be constructed from solid material(s) that have adequatethermal conductivity and other desirable thermal properties such as highvolumetric heat capacity to provide thermal mass for energy storage. TSU12 also may be constructed from material(s) capable of withstanding highpressure, in addition to possessing desirable thermal properties. Forexample, TSU 12 may be constructed from iron, steel, aluminum, anyalloys thereof, ceramic, fluid-filled rigid structure, or any othersuitable material(s). TSU 12 also may be constructed of a material fromwhich energy may be extracted as the material transitions from a liquidto a solid. For example, TSU 12 may be filled with molten aluminum thatis normally maintained at approximately 670° C., e.g., by heaters 16.When power is needed to be extracted from, e.g., a TACAS systememploying TSU assembly 10, the molten aluminum cools and starts tosolidify, thus releasing heat at a substantially constant temperaturethat is then used to heat fluid flowing through flow channels 14 a-d.

Cartridge heaters 16 preferably are disposed in bores uniformlydistributed throughout TSU 12 such that the heaters provide uniformheating and allow for loss of an individual heater without significantloss in overall uniformity of temperature in TSU 12. This configurationalso reduces standby losses since most of the heat emanating from thecartridge heaters flows directly into and through the thermal storagemass of TSU 12 before passing through the insulation to the ambientenvironment.

The bores are sized such that there is a small air gap (preferably, lessthan 1 mm) between each heater and the perimeter of the bore. Becausethe air gap is so small, the primary modes of heat transfer between theheaters and the TSU are conduction and radiation. Advantageously, theair gap permits the heaters to be easily removed from the bores. If thenominal dimensions of the bores were approximately equal to that of theheaters, it may be difficult to remove the heaters from the bores forreplacement or repair if oxidation of the heater and bore surfacescauses the two structures to oxidize together. A small percentage of theheat emanating from the heaters may be lost to the ambient environmentwithout flowing through the thermal storage mass of TSU 12 by (1)escaping through the gap between the heaters and TSU 12 and/or (2) beingconducted down the sheath of heater 16 and lost through the end ofheater 16 that is adjacent to the insulation.

Preferably, heaters 16 are disposed within TSU 12 so that (1) theproximal ends of the heaters protrude out of the TSU and the insulation(not shown) surrounding the TSU and (2) the heater wiring may beconnected to a control system (e.g., control system 40 shown in FIG. 3)located external to the TSU. Preferably, heaters 16 are disposed in TSU12 so that each heater is independently removable from the TSU. That is,each heater may be physically detached from the TSU without having todetach any of the remaining heaters. Advantageously, this facilitatesrepair and replacement of failed heaters. Preferably, each cartridgeheater 16 is releasably coupled to a metal skin that covers theinsulation surrounding TSU 12 using fasteners 22, e.g., clips that screwinto studs that are affixed to the metal skin. In the embodiment of FIG.1, cartridge heaters 16 are disposed between flow channels 14a,c and14b,d and have longitudinal centerlines disposed orthogonal to thelongitudinal axis of TSU 12 such that the longitudinal centerlines ofthe heaters form a plane. Alternatively, cartridge heaters 16 may bedistributed throughout TSU 12 in an alternative configuration.

FIG. 2 illustrates a typical construction of an electric resistancecartridge heater 16 (as described in Integrating Electrical HeatingElements in Appliance Design; Hegbom, Thor, 1997 (page 306)). Cartridgeheater 16 comprises outer tube 24 made from a good thermal conductorsuch as metal. Inside outer tube 24 are two layers of insulators:cylindrical insulator 26 made from, e.g., ceramic, and insulation powder28 made from, e.g., magnesium oxide powder, disposed between ceramicinsulator 26 and outer tube 24. Insulators 26 and 28 may be made fromany electrically non-conducting materials having good heat transferproperties. Helically wound around the outer perimeter of insulator 26is resistive wire 30, which provides heat from cartridge heater 16 tothe TSU when current is run through the wire. The proximal end ofresistive wire 30 is coupled to terminal 32 a, which may be disposedthrough the longitudinal length of insulator 26. The distal end ofresistive wire 30 is coupled to terminal 32 b, which is disposed inelectrical isolation from terminal 32 a. Terminals 32 a-b protrude fromthe distal end of cartridge heater 16 through end plug 34. End plug 34prevents contaminants and moisture from entering outer tube 24. End plug34 may be made from any electrically non-conducting material. Electricalcartridge heaters are readily available from a number of manufacturers.Examples include the CIR Cartridge Heaters marketed by Chromalox®, Inc.of Pittsburgh, Pa., the Mighty Watt Cartridge Heaters marketed by OgdenManufacturing Co. of Arlington Heights, Ill., and the FIREROD® CartridgeHeaters marketed by Watlow Electric Manufacturing Company of St. Louis,Mo.

Advantageously, cartridge heaters are low cost, readily available, andwell characterized for performance and reliability. Cartridge heatersare available in a variety of lengths, diameters, powers and voltages.As opposed to radiant heaters, the use of cartridge heaters reduces thecost of the TSU assembly because it allows a manufacturer to takeadvantage of volume discounts. That is, a TSU system uses more cartridgeheaters per system than radiant heaters. This allows a manufacturer toqualify for volume discounts even when manufacturing fewer TSU systems.Furthermore, because cartridge heaters are compact heaters, thecartridge heaters may be installed through the insulation surroundingthe TSU so that the heaters substantially share the same volume occupiedby the TSU. This reduces the overall size of the TSU assembly, ascompared to radiant heaters that are installed external to the TSU andrequires additional insulation to reduce flow of energy to the ambientenvironment. The compact size of cartridge heaters also allows anoperator to easily remove and replace a heater without requiringadditional manpower and/or special tools.

Pursuant to one aspect of the present invention, TSU assembly 10comprises one or more redundant heaters, which are heaters in excess ofa minimum number of heaters needed to heat/reheat TSU 12 withinspecification. More specifically, the minimum number of heaters is thatquantity needed to raise a characteristic temperature of TSU 12 (e.g.,the average temperature of TSU 12) to a predetermined value in apredetermined amount of time when the minimum number of heaters areoperated at the maximum power permitted by a heater control program,e.g., controller 40 of FIG. 3. The redundant heaters shorten the timeneeded to reheat the TSU after a discharge event in which thetemperature of the TSU is reduced and enables the heating system tooperate within specification even with failure of one or more heaters.That is, the redundant heaters allow the system to operate continuouslywithout significant loss in the overall uniformity of the temperature ofthe TSU, even when one or more of the redundant heaters have failed.Furthermore, when more than the minimum number of heaters areoperational, the load on each individual heater is reduced, extendingits operational life.

The use of redundant heaters also permits replacement or repair offailed heaters to be deferred to routine TACAS maintenance intervals,rather than requiring immediate repair or replacement when an individualheater malfunctions. For example, if the TSU assembly incorporates fiveredundant heaters in addition to the minimum number of heaters needed toheat/reheat the TSU, the TSU system operates within specification untilmore than five of the total number of cartridge heaters fail.Accordingly, even when one or more of the heaters fail, TSU assembly 10continues to heat TSU 12 within specification, e.g., by heating TSU 12to and maintaining TSU 12 at a steady state temperature. Replacement offailed heaters during routine maintenance permits continuous operationbetween maintenance intervals and potentially extends the operationallife of the remaining heaters.

Heaters 16 may fail in one of two modes. First, heater 16 may experiencea short-to-ground type of failure, e.g., a failure that occurs when theinsulation within the heater fails. In a short-to-ground type offailure, the failed heater draws excessive amounts of current. Second,heater 16 may experience element failure by one of several causes,including fracture/breakage of resistive wire 30 at one or morelocations along its length. This may result from oxidation of the wiresurface and the associated loss of material as the wire elongates andcontracts during thermal cycling. Additionally, element failure canoccur when wire 30 breaks at a junction (weld joints, etc.) used toconnect adjacent sections of wire or when solder connections connectingterminals 32 a and/or 32 b to resistive wire 30 fail. In elementfailure, the failed heater acts as an open-circuit and stops drawingcurrent.

In one embodiment of the present invention shown in FIG. 3, the networkof cartridge heaters 16 are configured to allow any individual heater 16to be removed automatically from the electrical circuit after the heaterfails, either by a short-to-ground or element failure, without affectingoperation of the remaining heaters, thereby permitting uninterruptedoperation of the TSU system. In one embodiment, each cartridge heater 16is coupled in series to fuse 36 that is sized to rapidly disconnect itsassociated cartridge heater in the event of a short-to-ground type offailure. All heater-fuse assemblies are connected in parallel to form aheater network, which is coupled to single electrical supply line 38that is controlled by heater controller 40. When an element failureoccurs, the failed heater automatically is removed from the heaternetwork because the failed heater stops drawing current from electricalsupply line 38, acting as an open circuit. Similarly, when ashort-to-ground failure occurs, the fuse coupled in series to the failedheater blows, rapidly disconnecting the failed cartridge heater from theheater network.

This configuration allows any individual heater 16 to be removedautomatically from the electrical circuit after the heater fails withoutaffecting operation of the remaining heaters, thereby permittinguninterrupted operation of the TSU system. That is, this permitscontinued operation of the system within specification, e.g., heatingTSU 12 to and maintaining TSU 12 at a steady state temperature. Theability to disconnect failed heaters automatically is particularlyeffective when used in conjunction with redundant heaters to provideuninterrupted operation of the TSU to both reheat TSU 12 to and maintainthe temperature of TSU 12 at steady state conditions.

Current sensor 42, which is disposed in series with electrical powersupply line 38 and coupled to controller 40, senses the current drawn bycartridge heaters 16. When a heater fails and automatically disconnectsfrom the heater network, the current flowing through power supply line38 reduces by a proportional amount. Current sensor 42 senses thiscurrent reduction, thereby detecting when a cartridge heater has failed.Controller 40, which may comprise a computer or application-specificintegrated circuit (ASIC), responds to the signals output by currentsensor 42 by adjusting control parameters, adjusting the power providedto the heaters, and generating warnings and/or alarms for maintenance.Advantageously, this allows controller 40 to limit the maximumtemperature to which heaters 16 are heated, extending the operationallife of the heaters, reducing maintenance costs and improvingreliability. Current sensor 42 may comprise an open loop Hall effecttransducer, e.g., the HAL 50-S current transducer marketed by LEMComponents of Switzerland, or another type of current sensor known toone of skill in the art or otherwise.

The present invention may be used in many applications. FIG. 4Aillustrates one such application. More specifically, FIG. 4A shows aTACAS system 21 for providing output power utilizing TSU assembly 10 ofFIGS. 1 and 3, described above. For example, FIG. 4A may represent abackup energy system that provides backup power to a load in the eventof a disturbance in the supply of power from another power source (e.g.,utility power failure.)

The following discussion of TACAS system 21 is not intended to be athorough explanation of the components of a TACAS, but rather anillustration of how TSU assembly 10 can enhance the performance of aTACAS system. For a detailed description of a TACAS system, seecommonly-assigned, co-pending U.S. patent application Ser. No. assembly10/361,728, filed Feb. 5, 2003, which is hereby incorporated byreference herein in its entirety.

As shown in FIG. 4A, TACAS system 21 includes storage or pressure tank23, valve 25, TSU assembly 10, electrical input 27, turbine 29,generator 31 and electrical output 33. When electric power is neededfrom system 21, compressed air from pressure tank 23 may be routedthrough valve 25 to TSU assembly 10. TSU assembly 10 may heat thecompressed air before it is provided to turbine 29.

The hot air emerging from TSU assembly 10 may flow against the turbinerotor (not shown) of turbine 29 and drive turbine 29, which may be anysuitable type of turbine system (e.g., a radial-flow turbine). In turn,turbine 29 may drive electrical generator 31, which produces electricpower and provides it to electrical output 33.

Also shown in FIG. 4A is turbine exhaust 35 (e.g., the exhaust gasesemerging from turbine 29). Turbine exhaust 35 may be vented through anexhaust pipe (not shown), or simply released to recombine withatmospheric air.

Not only is system 21 advantageous because it uses a relativelyinexpensive and efficient TSU, it is also non-polluting. That isbecause, unlike conventional systems that use fuel-combustion systems toprovide hot air to the turbine, it does not require a fuel supply toheat the air that is being supplied to turbine 29. Instead, TSU assembly10 may be powered by electrical input 27 during standby operation, whichprovides the energy needed to heat the compressed air, while providingeffective pressure containment. System 21 therefore provides thebenefits of heating compressed air from pressure tank 23 before it issupplied to turbine 29, without producing the harmful emissionsassociated with combustion systems.

TACAS system 21 may also include control circuitry 37 which may becoupled to both TSU assembly 10 and electrical input 27. Controlcircuitry 37 may include controller 40 of FIG. 3. Control circuitry 37,along with electrical input 27, may therefore be used to monitor andcontrol the temperature of TSU assembly 10. As a result, the TSUassembly 10 may be heated to and maintained at a desired steady statetemperature.

Moreover, valve 25 may be coupled to piping (not shown) that bypassesTSU assembly 10 and feeds into turbine 29 along with the output from TSUassembly 10. By controlling the portion of the total compressed air flowthrough the TSU, the ratio of heated to non-heated air provided toturbine 29 may be modified, thereby providing another means forcontrolling the temperature of the air being supplied to the turbine. Amore detailed discussion of systems and methods for controlling thetemperature and pressure of fluid being provided to turbine 29 can befound, for example, in U.S. patent application Ser. No. ______, filedSep. 17, 2004 (Attorney Docket No. AP-48), entitled “Systems and Methodsfor Controlling Temperature and Pressure of Fluids” and U.S. patentapplication Ser. No. ______, filed Sep. 17, 2004 (Attorney Docket No.AP-50), entitled “Systems and Methods for Controlling Pressure ofFluids”, both of which are hereby incorporated by reference in theirentireties.

Another advantage of utilizing TSU assembly 10 is that larger pressuretanks are not required as is the case with compressed air storagesystems that do not utilize thermal storage units or combustion systems.

The present invention is presented in the context of industrial backuputility power. Alternatively, the present invention may be used in anyapplication associated with generating power, such as in thermal andsolar electric plants or continuously operating TACAS systems.Furthermore, the present invention may be used in any other applicationwhere thermal storage, fluid heating or heated fluid delivery may bedesirable.

FIG. 4B illustrates TSU assembly 10 (absent insulation) disposed withinsystem cabinet 46 for a backup generator 44. In a preferred embodiment,TSU assembly 10 is oriented for access to cartridge heaters 16 from thefront of system cabinet 46. In the embodiment of FIG. 4B, backupgenerator 44 includes compressor 48 disposed between TSU assembly 10 andcabinet door 50. To permit ease of access to TSU assembly 10 andcartridge heaters 16, compressor 48 is designed to be easily detachablefrom backup generator 44.

Referring now to FIGS. 5A-B, a preferred control algorithm forcontrolling cartridge heaters 16 is described. The control algorithmpreferably requires no more than a single temperature input signal tocontrol the electric power provided to heaters 16. Such a controlalgorithm is based, in part, on a single temperature input from the TSUand a characteristic heating curve. Preferably, the control algorithmaccepts TSU sensor temperature of FIG. SA and not the average heatertemperature of FIG. 5A because temperature sensors for the heater may bemore prone to failure than temperature sensors for the TSU due to thehigher heater temperatures. Instead, the control algorithm preferably isprogrammed to infer the average heater temperature based on the TSUsensor temperature and a model that provides a relationship, which maybe empirically determined or derived from thermal modeling, among theTSU sensor temperature, the average heater temperature, and optionallythe average temperature of the TSU (i.e., the TSU average blocktemperature in FIG. 5A). Using the model, characteristic heating curve Hof heaters 16, illustratively depicted in FIG. 5A, may be determinedgiven the behavior of the TSU sensor temperature. The temperature ofeach cartridge heater 16 theoretically equals the average heatertemperature shown in FIG. 5A.

As used herein, when controller 40 is programmed to require no more thana single temperature input to control the power provided to heaters 16,the controller is capable of controlling the heater power with one ormore input data signals, only one of which represents temperature.Accordingly, the controller still may accept one or more input datasignals in addition to a single input data signal that representstemperature. The controller may still receive input signals that arefunctions of other parameters, such as a signal indicative of thecurrent from current sensor 42 of FIG. 3. Indeed, controller 40 also mayaccept additional input signals that represent temperature so long asthe controller is capable of controlling the power delivered to theheaters using one or more data input signals, only one of whichrepresents temperature. For example, the controller may acceptadditional data signals representing temperature to implement a backupalgorithm to detect fault conditions (as described in greater detailhereinbelow).

In an alternative embodiment, the heater system of the present inventionmay be programmed to use a two temperature input control system thataccesses both the heater temperature and the temperature of the TSU forproper operation. Such a two temperature input control system may bemore complex to implement than a single temperature control system, suchas that described below with respect to FIGS. 5A and 5B. That is, in atwo temperature control system, the heater controller operates properlywhen both temperature signal inputs are accurate (within predeterminedtolerance levels). If one malfunctioned, then the control system maybecome non-operational. Since a TSU assembly requires a high degree ofreliability to service the TACAS system properly, the control system maybe programmed so that redundant temperature readings and algorithms forchecking the redundancies are implemented for each of the twotemperature inputs. Thus, the complexity of a reliable control systemincreases with the number of inputs. Furthermore, since sensors failmore rapidly at higher temperatures and thus temperature sensors for theheater may be more prone to failure than temperature sensors for theTSU, the control algorithm may be programmed with additional controlmeasures for the heater temperature sensors to provide an accuratecontrol signal.

Accordingly, while there may be advantages to using a control systemthat requires no more than a single temperature input signal to reliablycontrol the operation of the heater system, the heater system of thepresent invention alternatively may comprise a control algorithm thatrequires more than a single temperature input signal to reliably controlthe operation of the heater system. For example, two temperature inputsignals may be used with the variable time-base zero-crossing controlalgorithm discussed below with respect to CHART 1.

The TSU sensor temperature may be determined from a plurality oftemperature sensors, e.g., thermocouples 51 of FIG. 3, that aredistributed about the TSU preferably in thermally equivalent locations.The term “thermally equivalent locations” refers to isothermal locationsin the TSU in which all the sensors report the same temperature within aspecified tolerance. Preferably, controller 40 is programmed to use anaverage sensor temperature (which is designated as the TSU sensortemperature of FIG. 5A) to control the heating system of the presentinvention. This may be accomplished, for example, by couplingthermocouples in parallel so that the TSU sensor temperature representsthe average of the individual temperature data collected from thetemperature sensors. Indeed, when thermocouples disposed in thermallyequivalent locations are coupled in parallel, the average of thetemperature signals theoretically is equal to the temperature sensed byeach thermocouple (not accounting for tolerances). This average sensortemperature may not, however, be the same value as the averagetemperature of the TSU (as discussed in greater detail below).Alternatively, controller 40 may be programmed to accept all thetemperature sensor signals and use the average of two or more of thetemperature sensor signals to control the heating system of the presentinvention. For example, the control algorithm may be programmed with apolling algorithm that determines and rejects data outliers. If atemperature sensor fails and disconnects from the sensor network, thesystem continues to operate normally.

The control algorithm illustrated in FIGS. 5A-B comprises severalstages. Immediately following a discharge event, in which the TSU isreheated after a discharge of power from the TACAS, the controlalgorithm enters an equilibrate stage. In the equilibrate stage,cartridge heaters 16 are idle for a predetermined period of time toprotect the heaters by permitting temperature gradients in the TSU andheaters to level out.

Thereafter, the control algorithm enters a ramp stage, in which power isprovided to cartridge heaters 16. During the ramp stage, the level ofpower provided to cartridge heaters 16 ramps up to full power (that is,the maximum power permitted by the control algorithm) over apredetermined period of time to soft-start the heaters. The maximumpower permitted by the control algorithm is selected, in part, based ona goal to heat the TSU to the TSU set point within a predeterminedamount of time. Note that, depending on the locations in which thetemperature sensors are disposed within the TSU, the actual averagetemperature of the TSU may lag the TSU sensor temperature when the TSUis being reheated. This is due to the fact that the temperature sensorsmay not be distributed at, and therefore not account for, the colderextremities of the TSU. However, as shown in FIG. 5A, the TSU sensortemperature and the average temperature of the TSU converges to the sameTSU set point temperature at steady-state standby operation.

Once the cartridge heaters are ramped up to the maximum power permittedby the control algorithm, the heaters are maintained at full power untilthe average heater temperature reaches a maximum heater temperature, theselection of which is based, in part, on the following considerations: adesire for heaters that have long operational life, a desire for cheaperand smaller heaters, and metallurgical and other thermal instabilitiesthat occur at higher temperatures. Because controller 40 preferably isprogrammed to control the power provided to the heaters based on the TSUsensor temperature, and not the average heater temperature, the controlalgorithm infers that the average heater temperature has reached themaximum heater temperature based on the TSU sensor temperature and themodel described above. Accordingly, the heaters are maintained at fullpower until the TSU sensor temperature reaches a value corresponding tothe maximum heater temperature. Advantageously, by limiting the TSUsensor and heater temperatures, controller 40 reduces the likelihoodthat the TSU may be damaged by excessive stress during pressurizedoperation.

If redundant heaters are used, the full power period is shorter thanthat experienced when only the minimum number of heaters is used to heatTSU 12. Advantageously, this reduces load on each heater and lengthensthe operational life of all the heaters.

If the previous discharge event only partially discharged the energystored in the TSU, the TSU sensor temperature may reach the valuecorresponding to the maximum heater temperature before the cartridgeheaters are ramped up to the maximum power permitted by the controlalgorithm. In this situation, control algorithm begins to reduce therate at which power is delivered to the cartridge heaters without evermaintaining the heaters at full power.

Once the TSU sensor temperature has reached the value corresponding tothe maximum heater temperature, the power to the cartridge heaters isreduced at a rate that maintains the average heater temperature at themaximum heater temperature. The control algorithm calculates this ratebased on the model described above.

When the TSU sensor temperature increases to a steady state temperaturecalled the TSU set point in FIG. 5A, the control algorithm switches tosteady state temperature control to power the heaters sufficiently tomaintain the TSU sensor temperature approximately at the steady stateTSU set point. This results in a reduction in the rate at which power isprovided to the cartridge heaters as the heaters continue to deliverheat to the extremities of the TSU and to offset thermal losses to theambient environment. The heater temperature is allowed to reduce towardsthe TSU set point. After a period of time determinable from the model,the TSU is fully charged and the TSU sensor temperature and the TSUaverage block temperature have converged to the steady-state TSU setpoint. Thereafter, the controller provides sufficient standby power tothe heaters to maintain the heater and TSU sensor temperatures at thesteady state TSU set point temperature. Advantageously, when redundantheaters are used, each heater is heated to a lower temperature than thatrequired when only a minimum number of heaters are used to maintain TSU12 in standby mode. This lengthens the operational life of all theheaters.

In preferred embodiments, the heater controller uses a control algorithmthat reduces temperature excursions within the thermal storage mass ofthe TSU by adjusting the power provided to the heaters at a highfrequency, e.g., 60 Hz. Such control algorithms may include a variabletime-base zero-crossing algorithm for AC power or a DC voltage controlalgorithm for DC power. For example, in a variable time-basezero-crossing control algorithm, a silicon controlled rectifier (SCR)may be switched on to permit conduction of power to the heaters when theAC voltage signal crosses zero volts. The variable time base controlsthe proportion of time in which conduction is permitted to the time inwhich conduction is not permitted. In variable-time based control, thecontroller changes the time base according to the power requirement.CHART 1 below provides an example of variable time based control over avariable period: CHART 1 No 0 cycles conducting for a 1 cycle periodconduction 25% power 1 cycle conducting, 3 cycles non- conducting forthe 4 cycle period 50% power 1 cycle conducting, 1 cycle non-conductingfor the 2 cycle period 75% power 3 cycles conducting, 1 cycle non-conducting for the 4 cycle period Continuous 1 cycle conducting for the1 cycle period conductionAdvantageously, in variable time base control, the heaters are switchedon and off much more frequently than in fixed time based control.Because the heaters are switched on and off more frequently, the heatersexperience less temperature variations, thereby increasing operationallife.

In a DC voltage control algorithm, the controller provides predeterminedDC voltage levels depending on the percent power required. For example,with a 100V power supply, the controller may provide 86.6V for 75% powerand 70.7V for 50% power. The heater controller of the present inventionalso may use other control algorithms known to persons of skill in theart or otherwise.

Heater controller 40 also may be programmed with an optional backupalgorithm to detect fault conditions, in addition to its main controlalgorithm described above. The backup algorithm may be programmed tocalculate acceptable power and temperature parameters based onmeasurements of the energy delivered from the TSU during a previousdischarge event and an associated state temperature, such as the TSUsensor temperature at the end of the discharge state. For example, theenergy delivered from the TSU during a discharge event may be estimatedbased on the mass of the thermal storage material of the TSU, thespecific heat of the thermal storage material, and the TSU's change intemperature. Based on this estimation of the energy delivered during thelast discharge event, the backup algorithm may be programmed todetermine the desired state of the heating system from the model of theheating system, e.g., illustrated in FIGS. 5A and 5B. From the desiredstate of the heating system, the backup algorithm can determineacceptable temperature and power parameters, incorporating tolerancelevels suitable for the application for which the system is used. If thereal-time TSU sensor temperature or a secondary temperature inputobtained from the TSU and/or the heaters deviates from the acceptabletemperature parameters, and/or the real-time power deviates from theacceptable power parameters, controller 40 may activate a safe operationmode in which the controller reduces the amount of power delivered tothe heaters to a predetermined level. The controller then may direct theTSU system to operate at the reduced level until maintenance can beperformed to correct the fault condition. Similar to the main controlalgorithm described above, the backup algorithm also may employ variabletime-base zero-crossing control for AC power, DC voltage control for DCpower, or another control algorithm known to one of skill in the art orotherwise.

Pursuant to another aspect of the present invention, methods and systemsare provided for retrieving cartridge heaters from the TSU assembly whenthe TSU assembly is still hot, e.g., at the TSU set point temperature.FIG. 6 illustrates one embodiment of a removal tool comprising heaterpuller 52 and protective housing 54. Protective housing 54 includes aninsulated cylinder or other appropriate shape having sufficient lengthto completely surround cartridge heater 16. This reduces the chancesthat an operator accidentally contacts any of the hot surfaces of thecartridge heater when the heater is removed from the TSU assembly andtransferred to storage and transport container 56.

Container 56 includes protective case 58 lined with insulation, plate 60having one or more holes through which a plurality of cartridge heaters16 may be stored, cover 62 and carrying handle 64. Preferably,protective housing 54 and container 56 are designed to handle heatersthat have been heated to their operating temperature, e.g.,approximately 760° C. If the protective housing and container aredesigned to handle heaters heated to a temperature less than theiroperating temperature, additional time is needed to cool down the TSUbefore removing the heaters therefrom. The holes in plate 60 are spacedapart sufficiently to permit placement of cartridge heaters 16 thereinusing heater puller 52 and protective housing 54. Preferably, plate 60has a sufficient number of holes to accept a complete set of heaters 16.Plate 60 is mounted at a distance from the bottom of protective case 58to allow cartridge heaters 16 to rest on the bottom. Alternatively, eachcartridge heater 16 may incorporate a flange (see FIG. 7A) that isdisposed on the heater such that the distal end of cartridge heater 16clears the insulation liner on the bottom of protective case 58 when theflange is resting on plate 60.

Heater puller 52 includes a coupler (illustrative embodiments of whichare described with respect to FIGS. 7A-G) disposed on the distal end ofpuller 52 and actuator 70 disposed on the proximal end of puller 52. Thecoupler is a mechanism for releasably engaging heater puller 52 to theproximal ends of heater cartridges 16. Actuator 70 may be mechanicallycoupled to coupler 68 and actuated to engage heater puller 52 to anddisengage heater puller 52 from cartridge heater 16. The coupler alsomay comprise locking pliers, a locking ferrule, or a notched sleeve thatrotates and locks to a complementary feature on the proximal end ofcartridge heater 16. The coupler also may comprise other couplingmechanisms known to one of ordinary skill in the art or otherwise.

FIGS. 7A-G illustrate numerous embodiments of the heater puller of FIG.6. FIG. 7A illustrates a first embodiment of a heater puller. Plier-typeheater puller 80 comprises coupler 82 and actuator 83. Coupler 82includes two gripping surfaces 84 that conform to the shape of theproximal end of heater 16. Actuator 83 includes two handles 86a and 86b,each having a distal end that is coupled rigidly to one of the grippingsurfaces, and pivot 88 about which the handles rotate. When the proximalends of handles 86 are urged apart, handles 86 pivot about pivot 88 sothat gripping surfaces 84 also move away from each other. Similarly,when the proximal ends of the handles are urged together, so too are thegripping surfaces. Thus, when a heater is placed between the grippingsurfaces, heater puller 80 may be engaged securely to the heater bysqueezing the handles together. While the heater puller is engaged to aheater, an operator can pull the heater from or push a heater into aTSU.

Optionally, heater puller 80 also may comprise latching mechanism 90disposed on the proximal end of actuator 83 (see FIG. 7B). Latchingmechanism 90 comprises hook 92 pivotally coupled to handle 86a, releaselever 94 rigidly coupled to hook 92, spring 96 coupled to hook 92 (viarelease lever 94) and handle 86 a, and anchor 98 coupled to handle 86 b.Once gripping surfaces 84 are engaged to heater 16, latching mechanism90 securely maintains that engagement without additional action on thepart of the operator.

In operation, an operator squeezes handles 86 a and 86 b together toengage gripping surfaces 84 to heater 16. The operator then may engagehook 92 to anchor 98. Spring 96 imparts tension to hook 92 to keep thehook engaged to anchor 98, thereby preventing gripping surfaces 84 fromreleasing heater 16. When the operator is ready to release heater 16from heater puller 80, the operator may actuate release lever 94 againstthe spring force of spring 94 and disengage hook 92 from anchor 98. Thispermits the operator to urge handles 86 a and 86 b apart, thereby urginggripping surfaces 84 apart and disengaging the gripping surfaces fromheater 16.

FIGS. 7C and 7D illustrate a second embodiment of the heater puller.Ferrule-type heater puller 100 comprises sliding sleeve 102, ferrule 104disposed at the distal end of sliding sleeve 102, center rod 106, andgrips 108 that are mounted on compliant extensions 109 of center rod104. Center rod 106 and grips 108 may be advanced into and out of centerbore 110, which extends from the proximal end of sliding sleeve 102 tothe distal end of ferrule 104, to respectively close and open grips 108.To open grips 108 (as shown in FIG. 7D), center rod 106 is advancedtowards the distal end of sliding sleeve 102. The compliance ofextensions 109 allows a heater to be inserted between grips 108. Slidingsleeve 102 and ferrule 104 then may be actuated in the distal directiontowards heater 16 to close grips 108. Ferrule 104 engages compliantextensions 109, contracting the extensions (and thus grips 108) aroundheater 16 and thereby securely engaging the heater to heater puller 100.While the heater puller is engaged to the heater, an operator can pullthe heater from or push a heater into a TSU.

Optionally, heater puller 100 also may comprise locking clip 112 whichis configured to engage center rod 110. Once grips 108 are engaged toheater 16 by actuating sliding sleeve 102 in the distal direction sothat ferrule 104 engages extensions 109, locking clip 112 may beattached to the proximal end of center rod 106 protruding out of slidingsleeve 102. This prevents the center rod from sliding back into thesliding sleeve and thereby prevents grips 108 from disengaging heater16.

FIGS. 7E-F illustrates a third embodiment of a heater puller. Notch-typeheater puller 120 comprises actuator 122 and coupler 124 having one ormore L-shaped slots 126. Each slot 126 incorporates detent 128 to slidepast associated pin 130 disposed on the proximal end of heater 16. Thispermits pin(s) 130 to engage coupler 124 with reduced rotation. Whilethe heater puller is engaged to the heater, an operator can pull theheater from or push a heater into a TSU. FIG. 7F provides an end view ofheater 16 with two pins 130.

FIG. G illustrates a fourth embodiment of a heater puller. Hook-styleheater puller 140 comprises actuator 142 and hook coupler 144 disposedon the distal end of actuator 142. Hook coupler 144 is designed to beengaged to loop 146 disposed on the proximal end of heater 16. While theheater puller is engaged to the heater, an operator can pull the heaterfrom or push a heater into a TSU.

Heater puller 142 also may comprise locking rod 148 that is rotatablyand slidably disposed through retainers 150, which in turn may becoupled to handle 152 of actuator 142. In operation, once hook 144 isengaged to loop 146, an operator can lock hook 144 to loop 146 beactuating locking rod 148. In particular, the operator may actuatelocking rod 148 in the distal direction towards heater 16 until thedistal end of locking rod 148 engages proximal face 154 of heater 16.Alternatively, locking rod 148 also may engage a complementary featureon proximal face 148 designed to receive the rod. The operator then mayrotate locking rod 148 so that latch 156 integral thereto locks intoreceptor 158. Because the distal end of rod 148 is engaged to theproximal face of heater 16, hook 144 cannot accidentally disengage fromloop 146. Advantageously, when the distal end of locking rod 148 isengaged to heater 16, the looking rod also may be used to push theheater back into the TSU.

In operation, an operator can switch the TACAS system containing the TSUsystem of the present invention to a maintenance mode, during whichpower to cartridge heaters 16 is turned off and the TACAS system isoffline and unavailable to provide backup power. The operator candisconnect the electrical connections of cartridge heaters 16 anddisplace any other hardware disposed in front of cartridge heaters 16(e.g., compressor 48 of FIG. 4). Once the operator removes or loosensheater restraints 22, the operator can engage heater puller 52 to theproximal end of cartridge heater 16 that protrudes out from insulation72 of TSU assembly 10. After a secure engagement is made, the operatorcan slide protective housing 54 over the proximal end of heater puller52 and pull heater 16 from its bore in TSU 12 into protective housing54. Thereafter, heater 16 is deposited into storage and transportcontainer 56.

The operator then can slide a new heater 16 into the vacant borepreferably at a predetermined rate, secure the heater to TSU 12 usingthe associated heater restraint, and connect the heater to theelectrical supply. After the operator completes removal and installationof the desired number of heaters, heater controller 40 preferably isrestarted in a special restart mode similar to a reheat cycle describedabove with respect to FIGS. 5A-B. While the description herein describesshaft puller 52 as being detached from protective housing 54, it iswithin the scope of the invention to have a removal tool comprising anintegral shaft puller and protective housing.

If heaters 16 and thermal storage mass 12 are designed so that theproximal ends of heaters 16 do not protrude out of insulation 72, heaterpuller 52 may be designed to engage an engagement feature on theproximal face of heater 16.

Advantageously, the removal tool and storage/transport container enablesan operator to replace one or more heaters without having to wait theseveral days it typically takes for the TSU to cool to a temperature lowenough to permit the operator to handle the heater without protectiveequipment. This reduces the duration of the maintenance interval whenthe TACAS unit is offline and the backup power system is unable toprovide backup power.

Although illustrative embodiments of the present invention are describedabove, it will be apparent to one skilled in the art that variouschanges and modifications may be made without departing from theinvention. For example, while the present specification describe use ofresistive cartridge heaters, other electric resistance heaters suitablefor insertion into a thermal storage unit or any other types of heatersappropriate for the present invention also may be used. Furthermore, thepresent invention also may be used with three-phase power. In that case,a set of heaters 16 may be provided for each phase or heaters 16 maycomprise a plurality of three-phase heaters. If fuses 36 also areemployed, a fuse may be provided for each heater or phase of a heater.It is intended in the appended claims to cover all such changes andmodifications that fall within the true spirit and scope of theinvention.

1. A thermal storage system for heating fluid flowing therethrough, thesystem comprising: a thermal storage unit (TSU) having a firstlongitudinal axis; insulation at least partially surrounding the TSU; atleast one flow channel disposed within the TSU; an inlet in fluidiccommunication with the at least one flow channel, the inlet acceptingthe fluid to be heated; an outlet in fluidic communication with the atleast one flow channel; a plurality of heaters, each of the plurality ofheaters having a longitudinal centerline and a length, wherein theplurality of heaters are disposed at least partially within the TSU andeach of the plurality of heaters are independently removable from theTSU; and a controller for controlling electric power provided to theplurality of heaters.
 2. The system of claim 1, wherein the plurality ofheaters comprises a plurality of resistive cartridge heaters.
 3. Thesystem of claim 1, wherein the longitudinal centerline of each of theplurality of heaters is not parallel to the first longitudinal axis. 4.The system of claim 3, wherein the longitudinal centerline of each ofthe plurality of heaters is orthogonal to the first longitudinal axis.5. The system of claim 3, wherein the longitudinal centerlines of theplurality of heaters form a single plane that is parallel to the firstlongitudinal axis.
 6. The system of claim 1, wherein the at least oneflow channel comprises at least two flow channels each having a channelcenterline parallel to the first longitudinal axis, wherein the at leasttwo flow channels are disposed next to each other and the plurality ofheaters are disposed in between the at least two flow channels.
 7. Thesystem of claim 1, wherein the plurality of heaters are disposed toprotrude externally out of the insulation.
 8. The system of claim 1,wherein the plurality of heaters comprises one or more redundantheaters.
 9. The system of claim 1, wherein at least two of the pluralityof heaters are coupled in parallel to form a heater network, wherein theheater network is coupled to an electric power source.
 10. The system ofclaim 1, further comprising a plurality of heater-fuse assemblies eachhaving a fuse coupled in series with one of the plurality of heaters,wherein at least two of the plurality of heater-fuse assemblies arecoupled in parallel to form a heater network.
 11. The system of claim 9,further comprising a current sensor, wherein: the current sensor iscoupled to the heater network such that the current sensor detectscurrent drawn by the heater network, and the controller is programmed toadjust the electric power provided to the plurality of heatersresponsive to signals from the current sensor.
 12. The system of claim1, further comprising a plurality of temperature sensors disposed tosense the temperature of the TSU at a plurality of locations, whereinthe controller is programmed to require no more than a singletemperature input signal to control the electric power provided to theplurality of heaters, wherein the single temperature input signal isequal to the average value of two or more of the temperatures sensed bythe plurality of temperature sensors.
 13. The system of claim 12,wherein the plurality of locations are a plurality of thermallyequivalent locations.
 14. The system of claim 12, further comprising acurrent sensor, wherein: the current sensor is coupled to the pluralityof heaters such that the current sensor detects current drawn by theplurality of heaters, and the controller is programmed to adjust theelectric power provided to the plurality of heaters responsive tosignals from the current sensor.
 15. The system of claim 1, furthercomprising: a heater puller having proximal and distal ends, a couplerdisposed on the distal end and an actuator disposed on the proximal end,wherein: the coupler is configured to engage at least one of theplurality of heaters, and the actuation of the actuator engages thecoupler to at least one of the plurality of heaters.
 16. The system ofclaim 15, further comprising a locking mechanism that prevents thecoupler from disengaging from the at least one of the plurality ofheaters.
 17. The system of claim 15, wherein: the coupler comprises aplurality of gripping surfaces to engage at least one of the pluralityof heaters, and the actuator comprises a plurality of handles and apivot about which each one of the plurality of handles rotates, each oneof the plurality of gripping surfaces coupled to one of the plurality ofhandles.
 18. The system of claim 15, wherein: the actuator comprises asliding sleeve and a center rod slidably disposed within the slidingsleeve, the sliding sleeve having a distal sleeve end and the center rodhaving a distal rod end, and the coupler comprises a ferrule disposed onthe distal sleeve end, a plurality of compliant extensions disposed onthe distal rod end, and grips mounted on the plurality of compliantextensions, the ferrule engaging the plurality of compliant extensionswhen the sliding sleeve is actuated in a distal direction with respectto the center rod.
 19. The system of claim 15, wherein: at least one ofthe plurality of heaters comprises at least one pin, and the couplercomprises at least one L-shaped slot, the L-shaped slot configured toengage the at least one pin.
 20. The system of claim 15, wherein: atleast one of the plurality of heaters comprises a loop, and the couplercomprises a hook configured to engage the loop.
 21. The system of claim15, further comprising a protective housing having insulation, theprotective housing having a longitudinal length equal to at least thelength of each of the plurality of heaters.
 22. A backup energy systemcomprising: the thermal storage system of claim 1 for heating fluid; aturbine coupled to the thermal storage system for receiving the heatedfluid from the outlet, the heated fluid driving the turbine; and anelectrical generator for providing electric power when the turbine isdriven by the heated fluid.
 23. The backup energy system of claim 22,wherein the fluid is compressed gas, the backup energy system furthercomprising a compressed gas system to provide the compressed gas to thethermal storage system.
 24. The backup energy system of claim 22,further comprising at least one temperature sensor to sense thetemperature of at least one component of the thermal storage system,wherein the controller is configured to reduce the electric powerprovided to the plurality of heaters when the temperature of the atleast one component of the thermal storage system deviates from at leastone acceptable temperature parameter that is related to parametersmeasured during a previous discharge event.
 25. A thermal storage systemfor heating fluid flowing therethrough, the system comprising: a thermalstorage unit (TSU) having a first longitudinal axis; insulation at leastpartially surrounding the TSU; at least one flow channel disposed withinthe TSU; an inlet in fluidic communication with the at least one flowchannel, the inlet accepting the fluid to be heated; an outlet influidic communication with the at least one flow channel; a plurality ofheaters; a controller for controlling electric power provided to theplurality of heaters; and a plurality of heater-fuse assemblies eachhaving a fuse coupled in series with one of the plurality of heaters,wherein at least two of the plurality of heater-fuse assemblies arecoupled in parallel to form a heater network.
 26. The system of claim25, further comprising a current sensor, wherein: the current sensor iscoupled to the heater network such that the current sensor detectscurrent drawn by the heater network, and the controller is programmed toadjust the electric power provided to the plurality of heatersresponsive to signals from the current sensor.
 27. A thermal storagesystem for heating fluid flowing therethrough, the system comprising: athermal storage unit (TSU) having a first longitudinal axis; insulationat least partially surrounding the TSU; at least one flow channeldisposed within the TSU; an inlet in fluidic communication with the atleast one flow channel, the inlet accepting the fluid to be heated; anoutlet in fluidic communication with the at least one flow channel; aplurality of heaters; and a controller for controlling electric powerprovided to the plurality of heaters, wherein the controller isprogrammed to require no more than a single temperature input signal tocontrol the electric power provided to the plurality of heaters.
 28. Thesystem of claim 27, further comprising a plurality of temperaturesensors disposed to sense the temperature of the TSU at a plurality ofthermally equivalent locations, wherein the single temperature inputsignal is equal to the average value of two or more of the temperaturessensed by the plurality of temperature sensors.
 29. The system of claim27, further comprising a current sensor, wherein: the current sensor iscoupled to the plurality of heaters such that the current sensor detectscurrent drawn by the plurality of heaters, and the controller isprogrammed to adjust the electric power provided to the plurality ofheaters responsive to signals from the current sensor.
 30. A thermalstorage system for heating fluid flowing therethrough, the systemcomprising: a thermal storage unit (TSU) having a first longitudinalaxis; insulation at least partially surrounding the TSU; at least oneflow channel disposed within the TSU; an inlet in fluidic communicationwith the at least one flow channel, the inlet accepting the fluid to beheated; an outlet in fluidic communication with the at least one flowchannel; a plurality of heaters including at least one redundant heater;and a controller for controlling electric power provided to theplurality of heaters.
 31. A method for heating fluid flowing through athermal storage system, the method comprising: providing a thermalstorage unit (TSU) having at least one flow channel disposed therein;providing a plurality of heaters disposed at least partially within theTSU; controlling electric power provided to the plurality of heaters;transferring heat from the plurality of heaters to the TSU from aplurality of locations within the TSU; heating the TSU to a steady statetemperature within a predetermined amount of time; maintaining the TSUat the steady state temperature; transferring heat from the TSU to thefluid flowing in the at least one flow channel; and removing at leastone of the plurality of heaters from the TSU without removing theremaining ones of the plurality of heaters from the TSU.
 32. The methodof claim 31, wherein providing a plurality of heaters comprisesproviding a plurality of resistive cartridge heaters.
 33. The method ofclaim 31, wherein heating the TSU comprises heating the TSU to thesteady state temperature within the predetermined amount of time evenwhen one of the plurality of heaters fails.
 34. The method of claim 31,wherein controlling electric power further comprises requiring no morethan a single temperature input signal to control the electric powerprovided to the plurality of heaters.
 35. The method of claim 34,further comprising sensing current drawn by the plurality of heaters,wherein controlling electric power further comprises controllingelectric power provided to the plurality of heaters responsive to thesensed current.
 36. The method of claim 31, further comprising removingat least one of the plurality of heaters from the TSU while thetemperature of the TSU is substantially equal to the steady statetemperature.
 37. The method of claim 31, wherein controlling electricpower comprises controlling electric power provided to the plurality ofheaters using a DC voltage control algorithm.
 38. The method of claim31, wherein controlling electric power comprises controlling electricpower provided to the plurality of heaters using a variable time-basezero-crossing control algorithm.